In the cementing of well casings in well bores, it is the practice to employ one or more so-called cementing plugs to provide a barrier between the fluid within the well casing and a quantity of cement which is displaced into the casing, as well as to form a barrier between the cement and a following fluid employed to force the cement downwardly through the casing and thence upwardly through the annular space defined between the casing the well bore on the one hand, or, on the other hand, in the case of multiple strings of casing, in the annular space defined between the string of casing being cemented within the well bore and a previously set string of casing.
Customarily, the cementing of casing strings in well bores moreover has involved the use of so-called cementing heads which are devices adapted to be connected to the upper end of the casing string and providing a support for releasably holding the cementing plugs in the cementing head until it is desired that the plugs be released ahead of and behind the cement slurry as it is displaced through the cementing head into the well casing.
Such cementing heads are expensive to manufacture and maintain and, moreover, are subject to failure following repeated uses. Accordingly, the cost of the usual cementing head equipment is a significant factor in the total cost of a well cementing operation. Furthermore, in the use of cementing heads in the cementing of wells drilled from a platform on a body of water where the wellhead equipment, for the purpose of drilling, completing and producing the well, is all located at the bottom of the water, the cementing head may be so constructed that it constitutes a structural element which, during the running of the casing string into the well bore, must support the weight of the casing string. The weight of a string of casing may be substantial, in view of the fact that the casing may be of various diameters and wall thicknesses and may extend thousands of feet into the earth.